I have worked with many students who received an accommodation of extra time on their GMAT exam.
What I can tell you is this:
1) It is usually a lengthy process. Many if not most of the students I have worked with have been denied the first time by GMAC and have had to appeal. This process takes months.
2) Extensive documentation seems to be required, most students have information not just from a doctor but usually from the university that he or she attended as well.
3) From what I understand most people that get an accommodation had one throughout his/her college career.
Basically, if you were able to make it through college without an accommodation then you may need to make it through the GMAT without one.
I am not trying to discourage you, I am just saying that based on the aggregate of experiences of many students you should expect that it will take time (months) and that the process may be difficult and can end unsuccessfully.
And one more thing that I have observed in my experience, an accommodation is not nearly the advantage that people think it is. Because so many people are pressed for time during the exam they believe that extra time = a higher score. It is not necessarily true. The GMAT is constructed in such a way that unless you truly have something like dyslexia that can slow you down so much, extra time is not that helpful.
That being said, I do not speak for GMAC and I am not a doctor, and if you have a condition that is impacting you on a daily basis you should certainly consider seeing a doctor so that you can get help in terms of focusing. If you are not impacted on a daily basis and are just having trouble with the GMAT then it may turn out that you end up needing to focus on your GMAT techniques. Everyone has trouble focusing during the GMAT and there are many things you can do to improve that.
Do contact GMAC as others have suggested, and best of luck with whatever you decide.
